Method for Combination Solid State and Molten Salt Tantalum Reduction

ABSTRACT

A two-phase reduction method for producing tantalum powder includes loading an unheated reaction vessel with a layer of K 2 TaF 7 , then a layer of solid sodium metal, and then followed by a layer of solid diluent salt. A first heating phase is used to promote the solid state reduction of the layer of K 2 TaF 7 , which results in the production of very fine tantalum particles while minimizing tantalum crystal growth. A second heating phase is then used to melt the contents of the reactor vessel and react primary quantities of sodium metal and K 2 TaF 7  to produce tantalum powder. In certain embodiments, the fine tantalum particles produced during the first heating phase serve as the nucleation sites needed for tantalum crystal growth in the second heating phase.

FIELD OF THE INVENTION

The present disclosure relates generally to tantalum reduction products and, more particularly, to a method for the reduction of tantalum using a combination solid state and molten salt tantalum reduction so as to control undesirable crystal growth in order to optimize the particle size distribution of tantalum powder.

BACKGROUND OF THE INVENTION

With evolution and microminiaturization of microprocessors and other electronics, the need for high-capacitance tantalum capacitors has rapidly increased. Moreover, the demands on purity and surface area for use in electrolytic capacitors are very high.

In order to satisfy the need in the art for high-purity tantalum powder, potassium tantalum fluoride (K₂TaF₇) is generally reduced using sodium metal so as to form tantalum powder. One of the key qualities of tantalum powder is that it should have a very narrow particle size distribution. The optimum median particle size is a function of the intended formation voltage of the capacitor in which the tantalum powder is to be used. However, one of the problems with producing tantalum powder having a uniform particle size is that a sodium reduction reaction of K₂TaF₇ is highly exothermic, with the reaction rate increasing as temperature increases. This is significant since both temperature and reaction rate tend to have a negative effect on the uniformity of tantalum crystal growth, which in turn directly affects the uniformity of the resulting tantalum particles produced.

In an attempt to control particle size uniformity, prior art techniques for reducing K₂TaF₇ using metallic sodium have included specific sequences of the introduction of the reactants into the reactor vessel. For example, some prior art techniques include preloading the reactor with molten K₂TaF₇ followed by adding molten sodium. Additionally, reactors have been preloaded with molten sodium followed by the addition of solid K₂TaF₇, or mixing solid K₂TaF₇ with solid sodium and loading this mixture into a reactor followed by heating of the mixture until a sustained chemical reaction is initiated.

However, none of the prior art techniques are able to adequately control crystal growth such that the particle size distribution for the tantalum powder is optimized. Thus, it is generally desirable to provide an improved method for the reduction of tantalum using a combination solid state and molten salt tantalum reduction so as to control undesirable crystal growth in order to promote uniform tantalum powder particle size.

SUMMARY OF THE INVENTION

Disclosed and claimed herein are methods for producing tantalum powder. In one embodiment, a method includes loading an unheated reactor vessel with a layer of solid K₂TaF₇, loading the unheated reactor vessel with a layer of solid sodium metal over the layer of solid K₂TaF₇, and then loading the unheated reactor vessel with a layer of solid diluent salt over the layer of solid K₂TaF₇ and the layer of solid sodium metal. The method further includes heating the reactor vessel to a first phase temperature to promote a solid state reduction of the layer of K₂TaF₇, and then heating the reactor vessel to a second phase temperature sufficient to melt the contents of the reactor vessel. Primary quantities of sodium metal and K₂TaF₇ are then added to the reactor vessel so that they may react to produce tantalum powder.

Other aspects, features, and techniques of the invention will be apparent to one skilled in the relevant art in view of the following detailed description of the invention

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawing in which like reference characters identify correspondingly throughout and wherein:

FIG. 1 is a process for performing a combination solid state and molten salt tantalum reduction in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The disclosure relates generally to a novel method for reducing K₂TaF₇ using sodium metal so as to form tantalum powder. In one embodiment, a method for the production of tantalum powder is provided in which crystal growth is controlled in a manner which optimizes the uniformity of the resulting tantalum particles produced. As will be described in more detail below, this may be accomplished using a combination solid state and molten salt tantalum reduction. In particular, a two-phase reduction method is disclosed in which primary nucleation is controlled as a separate event from the primary sodium reduction of K₂TaF₇, thereby improving tantalum crystal size and growth, and improving the electrical performance of the final tantalum powder.

Referring now to FIG. 1, depicted is a process for performing a combination solid state and molten salt tantalum reduction in accordance with one embodiment of the invention. In particular, process 100 begin at block 110 where a cold reactor vessel is loaded with a layer of solid K₂TaF₇. While in one embodiment, the reactor vessel may be loaded at block 110 at approximately ambient temperature, the term “cold reactor” may refer to a reactor vessel that is at any temperature below approximately 100° C. As this material is intended to serve an initial loading function, the amount of solid K₂TaF₇ loaded into the reactor vessel at block 110 may be on the order of between about 1 Kg and about 200 Kg. The dimensions and materials of construction for the reactor vessel are known and beyond the scope of this disclosure.

Once the reactor vessel is loaded with a layer of solid K₂TaF₇, process 100 may continue to block 120 where a layer of solid metallic sodium is added to the cold reactor over the existing layer of K₂TaF₇. Again, since this material is intended to serve an initial loading function, the amount of metallic sodium to be added at block 120 may be on the order of between about 0.5 Kg and about 100 Kg. In another embodiment, the amount of metallic sodium to be added at block 120 may be some amount at or above the amount needed in order to have an excess sodium environment following a full reaction with the K₂TaF₇ added at block 110. That is, the amount of metallic sodium to be added at block 120 may be no less than the amount needed to stoichiometrically react all of the tantalum ion (Ta +5) in the previously-added K₂TaF₇. This arrangement tend to suppress undesired crystal growth during the solid state reaction phase, as described herein.

Thereafter, a layer of solid diluent salts may be added to the reactor vessel over the existing layers added at each of blocks 110 and 120. While in certain embodiments the diluent salts may comprise NaCl, KCl, KF and/or NaF, it should equally be appreciated that any non-reactive substitute may be similarly used. At this point, the reactor vessel will contain three layers—the layer of K₂TaF₇ on the bottom, followed by the layer of sodium metal, which is in turn covered by the layer of solid diluent salts.

Continuing to refer to FIG. 1, process 100 may then continue to block 140 at which point the first phase of heating is commenced. In one embodiment, the first phase of heating involves the gradual heating of the reactor vessel until a temperature of between about 200° C. and 300° C. is achieved. At about 100° C. the sodium metal will melt, but the diluent salts and K₂TaF₇ will remain in the solid state. Once the reactor vessel reaches the first phase target temperature (i.e., between about 200° C. and 300° C.), the initial solid state reaction between the sodium metal and K₂TaF₇ will begin. In order to avoid hot spots where premature melting may occur, the heating rate may be controlled to be less than 10° C. per minute.

While this reaction is exothermic in nature, it will tend to proceed in a controlled manner since the reactor temperature is relatively low, the K₂TaF₇ is still in the solid state, and the diluent salts covering the K₂TaF₇ and metallic sodium layers will tend to control the reaction rate. In addition, the fact that the reduction reaction is being highly controlled results in beneficially prohibiting significant crystal growth from taking place. As a result of the lack of significant crystal growth, the tantalum particles which are produced during the slow solid state reaction of the first heating phase will tend to be submicron in size and numerous in number. In particular, the tantalum particles formed during the first phase will tend to be discrete and generally range between about 0.1 and about 0.5 microns in size. Moreover, and as previously mentioned, since the reaction vessel has been loaded with excess metallic sodium, there will tend to be little or no un-reacted tantalum ions which may otherwise promote undesirable crystal growth.

Process 100 may then continue to block 150 where the second phase of heating may be commenced. While in the embodiment of FIG. 1 the second heating phase is carried out in the same reactor vessel as the first heating phase, it should equally be appreciated that different reactor vessels similarly may be used. In particular, once the first heating phase has been completed using a first reactor vessel, the solid state reaction products in that first reactor vessel may be transferred to a second reactor vessel in which the second heating phase may then be carried out. This may be desirable since the design of the reactor vessels may be optimized for a particular set of reaction conditions.

Regardless of whether one or two reactor vessels are used for the first and second phases, the second phase of heating may involve the heating of a reactor vessel to a temperature of between about 650° C. and 1000° C. In certain embodiments, the second heating phase may comprise heating the reaction vessel to an internal temperature that is sufficient to melt the contents in the reactor vessel. While K₂TaF₇ itself melts at 720° C., the actual melting point of the overall mixture of metallic sodium, K₂TaF₇ and diluent salts in the reaction vessel may range (e.g., from 650° C. to 900° C.). It should also be appreciated that heating the reaction vessel to greater than 1000° C. is not inconsistent with the principles of the invention, but may be considered suboptimal due to practical limitations on equipment, higher energy cost and the like.

Once the temperature required to fully melt the K₂TaF₇ mixture has been achieved, process 100 may then proceed to block 160 where the primary raw materials of sodium metal and K₂TaF₇ may be added to the reaction vessel. Unlike the small quantities of sodium metal and K₂TaF₇ previously added at blocks 110 and 120, the sodium metal and K₂TaF₇ to be added at block 160 represent the vast majority of the actual raw materials that will be consumed during the primary reduction reaction. The sodium metal and K₂TaF₇ added to the reactor vessel at block 160 during the second phase may be added in any combination and in any number of incremental amounts.

It should be appreciated that, prior to or in conjunction with the addition of the primary sodium metal and K₂TaF₇ at block 160, the reactor contents may be mechanically agitated so as to further disperse the tantalum particles created during the first phase. While a standard bowtie agitator may be used, it should equally be appreciated that any known means of agitating reactor vessel contents may be similarly employed.

After the reactor vessel contents have melted and been optionally agitated, the fine and uniform tantalum particles created during the first phase will serve as the primary nucleation sites for the reduction and crystal growth during the second heating phase. Since crystal growth was suppressed during the first phase, the discrete tantalum particles that were produced during the first phase were fine and uniform in size. The vast number and small size of those tantalum particles produces a rich environment of uniform nucleation sites for tantalum powder production during the second phase. In this fashion, the two-phase reduction method of the present disclosure controls the primary nucleation as a separate event from the primary tantalum powder production which improves tantalum crystal size and growth, thereby beneficially affecting the electrical performance of the final tantalum powder.

After all reactants have been added, the reaction vessel may be allowed to cool and processed according to conventional methods know in the art. The resulting tantalum powder may then be used in the construction of capacitor anodes using known capacitor manufacturing techniques.

The following examples are provided to further illustrate the exemplary embodiments and do not limit the scope of the invention:

EXAMPLE 1

At ambient temperature, an empty reactor vessel with approximately 100 gallons capacity is pre-loaded with 1.0 Kgs of K₂TaF₇ powder as a single pile in the center-bottom of the vessel. On top of this layer of K₂TaF₇ is placed a solid 1.0 Kg ingot of sodium metal also at room temperature. This composite is then buried under 300 Kg of KCl. For the first heating phase, the reactor is closed and the air replaced with argon per typical reduction procedures and heated to 300° C. degrees at a rate of 1 degree Celsius per minute, and then held at 300° C. for 60 minutes. This arrangement results in a solid state reduction reaction between the metallic sodium and K₂TaF₇. The amount of sodium metal used in this phase is in excess of the stoicheometric amount needed to completely reduce the 1.0 Kg of K₂TaF₇, thereby minimizing the rate of crystal growth of the submicron tantalum particles formed during this first phase.

For the second heating phase, the sealed reactor is then further heated to 800° C. and agitated after the bulk of the KCl has melted. The very fine tantalum particles produced in the first heating phase are therefore stirred up in the molten salt to act as the nuclei of the subsequent primary reduction. Into this stirred molten salt is added 100 Kgs of K₂TaF₇, which is allowed to melt into the stirred KCl. Into this mixture is then added 29.3 Kgs of molten sodium at a rate of 0.2 Kg/min while maintaining the reactor temperature between 800° C. and 900° C. The product of this reduction is then recovered and processed through water and/or acid treatments as known to those in the art of tantalum reductions.

EXAMPLE 2

A reduction is performed in a similar fashion as in Example 1, except that in this Example 2 the 1.0 Kg of solid K₂TaF₇ and 1.0 Kg ingot of sodium metal are combined in a 5-inch diameter by 12-inch long steel reactor vessel fitted with a funnel-shaped lid topped with a 2-inch ball valve. This vessel is placed into a hot wall tube furnace and heated to 300° C. following a similar procedure as the first heating phase of Example 1. After holding at 300° C. for 60 minutes, the vessel is removed from the tube furnace, up-ended and attached by a pipe union to a 2-inch pipe nipple penetrating the lid of an empty, cold conventional sodium reduction reactor. The ball valve built into the lid of the small reactor is opened allowing the product of the solid state reduction to fall from the small vessel through the pipe nipple into the conventional reactor. Thereafter, 300 Kgs of KCl are poured into the conventional reactor and the reactor is heated under argon until the KCl is melted at approximately 800° C. for the second heating phase. The molten reduction phase is conducted as in Example 1 by the addition of 100 Kgs of K₂TaF₇ and 29.3 Kgs of molten sodium metal. Recover The product of this reduction is then recovered and processed through water and/or acid treatments as known to those in the art of tantalum reductions.

EXAMPLE 3

At ambient temperature, an empty reactor vessel with approximately 100 gallons capacity is pre-loaded with 100 Kgs of K₂TaF₇ powder as a single pile in the center-bottom of the vessel. On top of this layer of K₂TaF₇ is placed a solid 30 Kg ingot of sodium metal also at room temperature. As with Example 1, this composite is then buried under 300 Kg of KCl. For the first heating phase, the reactor is closed and the air replaced with argon per typical reduction procedures and heated to 300° C. degrees at a rate of 1 degree Celsius per minute, and then held at 300° C. for 60 minutes. This arrangement results in a solid state reduction reaction between the metallic sodium and K₂TaF₇. The amount of sodium metal used in this phase is in slightly excess of the stoicheometric amount needed to completely reduce the 100 Kg of K₂TaF₇, thereby minimizing the rate of crystal growth of the submicron tantalum particles formed during this first phase.

For the second heating phase, the sealed reactor is then further heated to 800° C. and agitated after the bulk of the KCl has melted. The very fine tantalum particles produced in the first heating phase are therefore stirred up in the molten salt to act as the nuclei of the subsequent primary reduction. Into this stirred molten salt is added 100 Kgs of K₂TaF₇, which is allowed to melt into the stirred KCl. Into this mixture is then added 29 Kgs of molten sodium at a rate of 0.2 Kg/min while maintaining the reactor temperature between 800° C. and 900° C. The product of this reduction is then recovered and processed through water and/or acid treatments as known to those in the art of tantalum reductions.

As used herein, the terms “a” or “an” shall mean one or more than one. The term “plurality” shall mean two or more than two. The term “another” is defined as a second or more. The terms “including” and/or “having” are open ended (e.g., comprising). The term “or” as used herein is to be interpreted as inclusive or meaning any one or any combination. Therefore, “A, B or C” means “any of the following: A; B; C; A and B; A and C; B and C; A, B and C”. An exception to this definition will occur only when a combination of elements, functions, steps or acts are in some way inherently mutually exclusive.

Reference throughout this document to “one embodiment”, “certain embodiments”, “an embodiment” or similar term means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of such phrases or in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner on one or more embodiments without limitation.

While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art. Trademarks and copyrights referred to herein are the property of their respective owners. 

1. A method for producing tantalum powder comprising: loading an unheated first reactor vessel with a layer of solid potassium tantalum fluoride (K₂TaF₇); loading the unheated first reactor vessel with a layer of solid sodium metal over the layer of solid K₂TaF₇; loading the unheated first reactor vessel with a layer of solid diluent salt over the layer of solid K₂TaF₇ and the layer of solid sodium metal; heating the first reactor vessel to a first phase temperature to promote a solid state reduction of the layer of K₂TaF₇; heating a second reactor vessel to a second phase temperature sufficient to melt the previously-loaded K₂TaF₇, solid sodium metal and solid diluent salt; adding primary quantities of sodium metal and K₂TaF₇ to the reactor vessel; and reacting the primary quantities of sodium metal and K₂TaF₇ to produce tantalum powder.
 2. The method of claim 1, wherein the second reactor vessel is the first reactor vessel.
 3. The method of claim 1, wherein the second reactor vessel is different than the first reactor vessel, and wherein the method further comprises transferring, after said heating the first reactor vessel, the previously-loaded K₂TaF₇, solid sodium metal and solid diluent salt to the second reactor vessel.
 4. The method of claim 1, wherein the layer of solid K₂TaF₇ comprises between about 1 Kg and about 200 Kg of solid K₂TaF₇.
 5. The method of claim 1, wherein the layer of solid sodium metal comprises between about 0.5 Kg and about 100 Kg of metallic sodium.
 6. The method of claim 1, wherein the first phase temperature is between about 200° C. and about 300° C.
 7. The method of claim 1, wherein the second phase temperature is between about 650° C. and about 1000° C.
 8. The method of claim 7, wherein the second phase temperature is between about 650° C. and about 900° C.
 9. The method of claim 1, adding the primary quantities of sodium metal and K₂TaF₇ comprises incrementally adding sodium metal and K₂TaF₇ to the second reactor vessel.
 10. The method of claim 1, further comprising agitating the primary quantities of sodium metal and K₂TaF₇ in the second reactor vessel while heating the second reactor vessel to a second phase temperature.
 11. The method of claim 1, wherein loading the unheated first reactor vessel with the layer of solid sodium metal comprises loading the first reactor vessel with an excess quantity of solid sodium metal for complete reduction of the layer of solid K₂TaF₇.
 12. The method of claim 1, wherein the unheated first reactor vessel has an internal temperature of below approximately 100° C.
 13. The method of claim 1, wherein a plurality of discrete tantalum particles are produced as a result of the solid state reduction of the layer of K₂TaF₇, and wherein the plurality of discrete tantalum particles are each submicron in size.
 14. The method of claim 13, wherein reacting the primary quantities of sodium metal and K₂TaF₇ comprises forming tantalum crystals on a plurality of nucleation sites, wherein the plurality of nucleation sites comprises the plurality of discrete tantalum particles produced from the solid state reduction of the layer of K₂TaF₇.
 15. A two-phase method for producing tantalum powder comprising: loading an unheated reactor vessel with an initial quantity of solid potassium tantalum fluoride (K₂TaF₇); loading the unheated reactor vessel with an initial quantity of solid sodium metal; loading the unheated reactor vessel with an initial quantity of solid diluent salt; providing a first heating phase during which the reactor vessel is heated to a first phase temperature sufficient to promote a solid state reduction of the initial quantity of K₂TaF₇; providing a second heating phase during which the reactor vessel is heated to a second phase temperature sufficient to melt said initial quantities in the reactor vessel; and introducing a primary quantity of sodium metal and a primary quantity of K₂TaF₇ to the reactor vessel during the second heating phase, wherein the primary quantities of sodium metal and K₂TaF₇ react during the second heating phase to produce tantalum powder.
 16. The method of claim 15, wherein providing the first heating phase comprises providing the first heating phase in a first reactor vessel, and wherein providing the second heating phase comprises providing the second heating phase in a second reactor vessel.
 17. The method of claim 15, wherein the initial quantity of solid K₂TaF₇ comprises between about 1 Kg and about 200 Kg.
 18. The method of claim 15, wherein the initial quantity of solid sodium metal comprises between about 0.5 Kg and about 100 Kg.
 19. The method of claim 15, wherein the first phase temperature is between about 200° C. and about 300° C.
 20. The method of claim 15, wherein the second phase temperature is between about 650° C. and about 1000° C.
 21. The method of claim 20, wherein the second phase temperature is between about 650° C. and about 900° C.
 22. The method of claim 15, wherein introducing the primary quantities of sodium metal and K₂TaF₇ comprises incrementally adding sodium metal and K₂TaF₇ to the reactor vessel.
 23. The method of claim 15, further comprising agitating the primary quantities of sodium metal and K₂TaF₇ in the reactor vessel during the second heating phase.
 24. The method of claim 15, wherein the initial quantity of solid sodium metal comprises an excess quantity of solid sodium metal for complete reduction of the initial quantity of solid K₂TaF₇.
 25. The method of claim 15, wherein the unheated reactor has an internal temperature of below approximately 100° C.
 26. The method of claim 15, wherein a plurality of discrete tantalum particles are produced during the first heating phase, and wherein the plurality of discrete tantalum particles are each submicron in size.
 27. The method of claim 26, further comprising forming tantalum crystals on a plurality of nucleation sites during the second heating phase, wherein the plurality of nucleation sites comprise the plurality of discrete tantalum particles produced during the first heating phase.
 28. A capacitor component comprising the tantalum powder manufactured according to the method of claim
 1. 29. A capacitor component of claim 28, wherein said capacitor component comprises a capacitor anode. 